Hydrate

Hydrates ( altgr. ὕδωρ Hydor, water ') are the chemical substances generally containing water. The term is handled slightly differently in different areas of chemistry. In contrast to the hydrates are the anhydrides, compounds, which water has been removed.

Inorganic hydrates

Inorganic hydrates are solvates, ie they contain molecular water, such as water of crystallization. In addition to hydrated crystals are also complexes that have water ligands, called hydrates. Crystal Waterless substances are also referred to as anhydrates.

The crystal water in hydrates may be attached through hydrogen bonding or other intermolecular forces to specific ions, but it may also be the case that it is built mainly because thereby a more favorable crystal structure. In crystalline hydrates there are certain stöchiometische relationships between ionic compound and water of crystallization. In many cases, several different crystal hydrates with water amounts are possible, as are modifications of sodium carbonate with one, two, five, seven and ten equivalents of water of crystallization and the crystal- water -free sodium carbonate known.

Hydrates caused by hydration, usually during the crystallization from aqueous solutions. Crystal water rich hydrates, such as by heating, are dehydrated, so that either other, containing less water crystal hydrates occur or the anhydrous compound.

For the identification of hydrates in empirical formulas this is not set directly in the formula with added, but as a · n H2O at the end of the sum formula ( n is the amount of water of crystallization, which occurs in the crystal per formula unit ). CaSO4 · 2 H2O, for example, describes calcium sulfate with two equivalents of water of crystallization, which corresponds to gypsum. To name the name of the compound is a - xxxhydrat added, where xxx is a Greek word for the number equivalent number of crystal water. Gypsum is called systematic accordingly calcium sulfate dihydrate.

Organic hydrate

In organic chemistry hydrated compounds are referred to as hydrates. These are usually not compounds containing molecular water, but those in which water has been added by an addition reaction of a compound chemically. Examples are geminal diols or aldehyde hydrates, where it is hydrated aldehydes:

Most of aldehyde hydrates are unstable, exceptions are chloral hydrate, formaldehyde and ninhydrin.

There are also stable organic compounds containing water of hydration, such as the (R) -cysteine ​​hydrochloride monohydrate [L- cysteine ​​hydrochloride monohydrate; CAS-No. 7048-04-6 ], which is produced on an industrial scale.

Gas hydrates

Wherein gas hydrates in contrast to the crystal water-containing substances is not the water in the substance, but otherwise, a gaseous compound such as methane, hydrogen sulfide or carbon dioxide is trapped in ice. Here, a clathrate, an inclusion compound formed without great physical attraction forces between ice and gas. The best-known gas hydrate is methane hydrate.